1. Field of the Invention
The present invention relates generally to acoustic surface wave signal processors for processing electrical input signals and yielding modified output electrical signals and more particularly concerns such apparatus as delay lines, bandpass filters and pulse compression filters.
2. Description of the Prior Art
The surface wave devices capable of performing signal processing functions, such as bandpass filters and pulse compression filters, take several forms. Early designs utilizing surface waves generally employed multi-section interdigital transducers which give rise to a number of undersirable side effects such as wavefront distortion, multiple reflections, radiation, bulk wave generation, and variation of effective surface wave velocity across the array. These lead to spurious signals and investigations to minimize these effects led to designs which employ reflecting grooves oriented obliquely to the incident wave. In these designs, a reflecting groove or line reflects the surface wave incident thereto into some other acoustic track providing a relative delay of the reflected signal which depends upon the position of the line in the array. Arrangement of the lines is such that the reflections from adjacent lines will be in-phase in a given direction relative to the incident signal. In the simplest form, a single array of reflecting lines provides a processed signal at 90.degree. relative to the incoming wave.
These systems are analogous to delay line processors wherein each groove corresponds to a tap on the delay line. Weighting factors to provide the desired signal processing are established by reflections from the grooves which vary with the depth of each groove. Though the device provides a high performance processor, the fabrication is relatively complicated, requiring two separate operations; the application of metallized strips to the substrate to form the transducer pattern; and the etching of the grooves in the array which must be precisely oriented with that of the transducer pattern. Furthermore, to vary the depth of the grooves, they must be ion beam etched in a process that varies the etching time according to the desired depth. This process is not easily adapted to mass production and results in a rather high unit cost. The purpose of this invention is to provide a reflective array processor that maintains the high level of performance of the prior art and may be economically fabricated.